Modern exhaust-gas purification systems for internal combustion engines usually comprise a NOx storage catalytic converter which, when the internal combustion engine is in lean operating mode, temporarily stores the nitrogen oxides which are produced. However, the storage capacity of such NOx storage catalytic converters is limited, so the exhaust-gas purification system regularly has to switch over to a regeneration mode, in which the nitrogen oxides stored in the NOx storage catalytic converter are released and reduced to nitrogen. In this regeneration mode, the internal combustion engine is operated with a rich exhaust-gas mixture, by which means hydrocarbons are provided in the exhaust gas as reducing agents, which reduce the stored nitrogen oxides to harmless nitrogen.
A problem with these known exhaust-gas purification systems, however, is the fact that shortly after the switchover to the regeneration mode, a desorption peak occurs in the exhaust gas downstream of the NOx storage catalytic converter. This means that the nitrogen oxide concentration in the exhaust gas of the internal combustion engine downstream of the NOx storage catalytic converter rises sharply for a short period upon switchover to the regeneration mode, as the nitrogen oxides stored in the NOx storage catalytic converter have then already been released, but have not yet have been reduced to nitrogen. In conventional exhaust-gas purification systems, an unwanted emission of nitrogen oxides therefore occurs upon switchover to regeneration mode due to the desorption peak.
An exhaust-gas purification system for an internal combustion engine is also known from EP 1 118 756 B1 in which the NOx desorption peak is determined and taken into account in the controlling of the internal combustion engine. This patent specification, however, is aimed solely at optimizing regeneration, without affecting the desorption peak itself. Here, the desorption peak is therefore merely evaluated, but not reduced. Here, too, a desorption peak therefore occurs upon switchover to regeneration mode, which is accompanied by unwanted nitrogen oxide emissions.